1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, the image forming apparatus described in Japanese Patent No. 4057232 is known as such a kind of image forming apparatus. FIG. 28 is a cross section partially illustrating a sheet feeding tray 950 serving as a sheet bundle housing unit of an image forming apparatus described in Japanese Patent No. 4057232. The sheet feeding tray 950 houses a plurality of recording sheets P piled as a sheet bundle. A bottom plate 951 on which the sheet bundle is placed is provided with a partial opening 952. There is provided, under the bottom plate 951, an end sensor 954 including a swing member 953 swingable with a fixed end as an axis. A free end of the swing member 953 is a hemispherical sheet contact portion 953a and projects above the bottom plate 951 through the opening 952. When the sheet bundle placed on the bottom plate 951 presses downward the sheet contact portion 953a projecting above the bottom plate 951, the swing member 953 moves, along a swing direction, to a position at which the sheet contact portion 953a does not project above the bottom plate 951. When the sheet bundle is removed from the bottom plate 951, the swing member 953 moves, along the swing direction, to a position at which the sheet contact portion 953a projects above the bottom plate 951. The end sensor 954 detects presence or absence of a sheet bundle on the sheet feeding tray 950 based on such movement of the swing member 953.
To refill the sheet feeding tray 950 having a such configuration with a sheet bundle normally, an operator lowers a sheet bundle held in his/her hand, brings it into contact with a sheet placement surface of the bottom plate 951, and then slides the sheet bundle on the placement surface for alignment before leaving the hand from the sheet bundle. When the sheet bundle is placed on a placement surface area where the sheet bundle is not in contact with the sheet contact portion 953a, prior to the alignment, it is possible that the sheet bundle be caught on the sheet contact portion 953a when being slid, which may force the operator to perform placement operation again. When the sheet bundle is placed on the sheet contact portion 953a and then slid so as to prevent the sheet bundle from being caught, it is possible that the sheet bundle be damaged by rubbing with the sheet contact portion 953a. 
Therefore, there is need for an image forming apparatus that enables refilling of the sheet bundle housing unit with a sheet bundle without allowing the sheet bundle to be caught on the sheet contact portion or damaged by rubbing with the sheet contact portion.